1. Field of the Invention
The present invention relates to a valve timing control device of an internal combustion engine, which controls open/close timing of engine valves, such as intake and exhaust valves of the engine, in accordance with an operation condition of an associated motor vehicle.
2. Description of the Related Art
As is known in the art, Atkinson Cycle in internal combustion engines is of a cycle type that improves a thermal efficiency of the engine by increasing compression ratio under operation of the engine. For realization of the Atkinson Cycle, a valve timing control device is usually employed, by which a close timing (viz., IVC) of each intake valve is retarded relative to the bottom dead center (viz., BDC) of the piston thereby to increase a volumetric efficiency of each cylinder. In this case, it is necessary for the valve timing control device to much increase a valve operation angle (viz., an angle for which the intake valve opens) and to control the valve open/close timing to a retarded side.
However, when the engine is stopped, it tends to occur that due to positive/negative alternating torque applied to the camshaft, each intake valve is automatically returned to an angular position for the most-retarded phase. Thus, if, with each intake valve assuming such most-retarded phase position, the engine is subjected to a cold starting or the like, the compression ratio of the engine fails to have a satisfied value because of less inertia of intake air thereby to cause the air in the combustion chamber at the top dead center (viz., TDC) of the piston to fail to reach a satisfied temperature and cause increase of the torque load, which makes the engine starting difficult.
In order to solve the above-mentioned drawback, Japanese Laid-open Patent Application (Tokkai) 2000-250073 proposes, for improved engine startability under cold condition, a valve timing control device which, upon stopping of the engine, causes the close timing of intake valves to take a middle phase between the most-advanced and most-retarded phases. For keeping the close timing to such middle phase, a vane member coaxially fixed to a camshaft is locked at a corresponding angular position.